Hyperthyroidism in Cats

Hyperthyroidism in Cats

Overview of Feline Hyperthyroidism

Feline hyperthyroidism, also referred to as thyrotoxicosis in cats, is one of the most common endocrine disorders in cats. If your adult cat suddenly begins to lose weight despite a voracious appetite, he may have a hormone problem, specifically the hormone produced by the thyroid gland. You should probably get your pet to a veterinarian to be checked for feline hyperthyroidism.

Below is an overview of hyperthyroidism in cats followed by detailed in-depth information on the diagnosis and treatment options for this disease. 

Feline hyperthyroidism has become a widely recognized disorder in cats. It is caused by an unregulated overproduction of thyroid hormone by the thyroid glands, which is usually related to a benign enlargement (growth or tumor) of one or both thyroid lobes. Cancer of the thyroid gland is found in less than 2% of cats. This enlargement of the thyroid gland(s) is referred to as thyroid adenoma or thyroid adenomatous goiter. It is unknown what causes the thyroid to become enlarged.

The thyroid gland consists of two flat lobes shaped like a butterfly and located on either side of the trachea, or windpipe, just below the voice box. These lobes are flattened and cannot be easily palpated. The thyroid gland acts as the thermostat for the metabolic rate of the body, controlling how fast or slow the body functions. Hyperthyroidism can have effects on multiple organ systems, since the increased thyroid hormone levels increase the cat’s metabolic rate.

Hyperthyroidism is diagnosed in cats from age 4 to 20 + years, although this disease is usually diagnosed in older cats (95% are at least 8 years of age). There is no recognized breed or sex predilection for this disease, although purebred cats seem to be less likely to be hyperthyroid.

There is no known cause for hyperthyroidism although canned food and ectoparasiticide exposure has been theorized.

What to Watch For

The classic signs are:

  • Weight loss in spite of an increased appetite
  • Restlessness and/or hyperactivity
  • Increased activity levels or irritability

    Other signs include:

  • Increased thirst
  • Vomiting and or diarrhea
  • Decreased grooming activity
  • Seeking cool areas

    Please note approximately 10% of cats have what is referred to as “apathetic hyperthyroidism” with atypical symptoms. Clinical signs may include lethargy, weight gain and decreased appetite.

  • Diagnosis of Hyperthyroidism in Cats

    Diagnosis can be made by a simple blood test that measures the level of the thyroid hormone (T4).

    Your veterinarian may also perform other diagnostic tests to exclude other diseases, including:

  • Complete medical history and physical examination
  • Complete blood count (CBC) and serum chemistry profile
  • Thoracic radiographs
  • Blood pressure
  • Echocardiogram
  • Electrocardiogram (if an abnormal or irregular heart rate is suspected)
  • A T3 suppression test in hard-to-diagnose cases
  • In some cases a radionuclide scan (used in diagnosis for whole body or individual organ scanning)
  • Treatment of Hyperthyroidism in Cats

    Treatment is directed at controlling excessive secretion of thyroid hormones and can involve a variety of approaches depending on several factors. These include your cat’s overall health, availability of radioactive iodine therapy and cost considerations. There are three main methods of treatment:

  • Radioactive iodine therapy
  • Surgical removal of the abnormal thyroid lobes
  • Medical therapy with Tapazole® (methimazole) and beta-adrenergic blockers (such as atenolol) to reduce some of the symptoms of hyperthyroidism
  • Iodine restricted diet such as Hills y/d
  • Home Care and Prevention

    At home, be sure to administer any medications prescribed by your veterinarian. If your cat is taking Methimazole, a potential adverse effect is loss of appetite, which may be related to liver complications.

    There is no prevention because the cause is not known. However, examination of the thyroid area should be a regular part of any veterinary examination in older cats. If weight loss occurs in your older cat, your veterinarian may recommend a thyroid blood test to screen for this condition.

    In-depth Information on Feline Hyperthyroidism

    Hyperthyroidism is a very common disease in older cats and can produce a variety of symptoms. Although this disease has been reported in cats as young as six years of age, the majority of cases have been in cats over eight years of age. There is no apparent breed or sex tendency.

  • The most common complaints in cases of feline hyperthyroidism are weight loss, polyphagia (increased appetite) and hyperactivity. Other significant signs that have been seen, although reported less frequently, include: polyuria (increased volume of urination), polydipsia (increased thirst), regurgitation, vomiting, diarrhea, increased fecal volume and increased respiratory rate or labored respiration.
  • Increased thyroid hormone levels cause increased energy metabolism and heat production in virtually all body tissues, resulting in increased appetite, weight loss, muscle wasting and increased body temperature.
  • Increased levels of thyroid hormones also interact with the nervous system, causing hyperexcitability, nervousness and muscle tremors.
  • Rapid overeating may cause regurgitation; this is generally seen in multiple-cat households. Vomiting may also result from a direct action of increased thyroid hormone levels on the chemoreceptor trigger zone, a center in the brain that causes vomiting.
  • Intestinal hypermotility associated with hyperthyroidism results in increased frequency of bowel movements and diarrhea.
  • The exact cause of polyuria and polydipsia in the hyperthyroid cat is unknown. Some patients may have concurrent chronic renal failure, which is very common in older cats. Physiologic changes in the kidney, due to increased renal blood flow, may result in kidney inability to concentrate urine normally, resulting in increased production of dilute urine and increased water consumption.
  • Increased levels of circulating thyroid hormones, over a long period, can result in heart changes known as thyrotoxic heart disease. Cats with this condition may exhibit signs of heart failure, with resultant rapid respirations or difficult breathing patterns.
  • Diagnosis In-depth

    Diagnostic tests are needed to recognize feline hyperthyroidism and exclude all other diseases. Diagnostics should include a complete medical history and physical examination. Many cats with hyperthyroidism are old and may have concurrent medical problems that can affect the choice of therapy. Your veterinarian will most likely recommend the following diagnostic tests:

  • A complete blood count (CBC) to evaluate the patient’s red and white blood cell counts. It’s important to know these values prior to starting therapy, as the medical therapy (Methimazole) most commonly used to treat hyperthyroidism can, in rare instances, adversely affect the cat’s red cell or white blood cell count.
  • A serum chemistry profile is needed to evaluate multiple organ systems, including the liver and kidneys. It’s important to know the status of all body systems prior to instituting therapy for hyperthyroidism and to identify any concurrent condition that may complicate therapy.
  • A serum thyroid hormone (T4) level should be evaluated. The serum T4 (thyroxine) measures the level of circulating thyroid hormone in the bloodstream. This is the test most commonly used to determine whether the thyroid gland is hyperactive. The T4 is also monitored once therapy is instituted to determine whether the therapy is effective. Mild high normal elevation with minimal clinical signs may indicate repeating of the thyroid T4 in a few weeks.
  • Serum Free T4 by Equilibrium Dialysis (FT4ED) is used in cats with clinical signs of hyperthyroidism but a normal or high normal value. Elevation of the FT4ED in combination with a high normal T4 is considered diagnostic.
  • Chest X-rays should be taken to evaluate the heart and lungs. Heart enlargement can be associated with hyperthyroidism; if cardiac changes are identified on X-rays, further evaluation of the heart is indicated.

    Additional diagnostic tests may be recommended on an individual pet basis, including:

  • A T3 suppression test. This is a blood test that may be performed to evaluate a patient suspected of being hyperthyroid, but consistently has serum T4 levels that are within the normal range. The T3 Suppression test has largely been replaced by the Serum Free T4 by Equilibrium Dialysis (FT4ED) test.
  • A Technetium-99m scan to evaluate a cat suspected of being hyperthyroid, but with normal serum T4 levels. It is also used to determine if one or both glands are involved there. This test uses a short-acting radioactive isotope to identify abnormal, hyperactive thyroid tissue via a nuclear scan. This test has limited availability and it can be done only in those facilities licensed to handle radioactive materials. Accordingly, it is often a referral procedure.
  • Treatment In-depth

    Treatment for feline hyperthyroidism may include one or more of the following:

  • Medical therapy with Tapazole® (Commonly known as Methimazole). Methimazole is a drug that interferes with the production of thyroid hormone by the thyroid gland; it does not affect the actual physical structure of the thyroid nodule itself. There are other related drugs available in Europe that can be used for the same purpose.
  • Patients receiving Methimazole must have their serum thyroid hormone levels measured periodically. They must also be monitored for any adverse reactions to the Methimazole; these reactions can include drug- induced anemia, low white blood cell counts and liver injury.
  • Treatment with Methimazole must continue for the remainder of the cat’s life and the dosage may have to be adjusted periodically. Most cats tolerate therapy with Methimazole very well.
  • This therapy is also appropriate for cats with heart failure due to hyperthyroidism. This treatment, along with heart failure medication, controls the thyroid condition while the heart failure is “stabilized.”

    Therapy may include surgical removal of the abnormal thyroid lobes. For example:

  • For cats that don’t respond well to medical therapy, or when owners prefer not to medicate their cats on a daily basis, thyroidectomy (removal of the thyroid gland) is a viable alternative. The advantage of this approach is that surgical removal of the abnormal thyroid lobe or lobes is curative. However, since approximately 75 percent of hyperthyroid cats have adenomas involving both thyroid lobes, a bilateral thyroidectomy is required to control the hyperthyroidism.
  • Following bilateral thyroidectomy, daily supplementation with thyroid hormone is required.
  • In addition, there is a risk of damaging or removing all four of the parathyroid glands, which regulate calcium levels in the cat’s body and are adjacent to the thyroid gland. Damage to, or removal of, the parathyroid glands would result in serious medical consequences, requiring careful monitoring and therapeutic intervention.

    Therapy may include radioactive iodine treatment. For example:

  • Radioactive iodine therapy (I131) is another potential treatment for hyperthyroidism. The advantage of this therapy is that it is usually a one-time treatment, does not subject the cat to the risk of anesthesia and surgery, has no need for long-term medication and follow-up care is usually not required.
  • The disadvantage of this form of treatment is that it is available only at facilities that are licensed to handle radioactive isotopes, and hospitalization of the patient for a minimum of 7 to 10 days is usually required in most states. Radiation safety laws in each individual state dictate length of hospitalization; these laws require that the patient be kept in isolation until the radioactivity from the isotope that was administered has fallen to a certain level.
  • Another disadvantage of this form of therapy is cost; however, although the initial cost may be significantly more than medical therapy, over time it is a very cost-effective means of treatment since, after successful therapy, there is no need for daily long-term medication or frequent visits to the veterinarian to monitor the patient.

    Other medical treatments may include:

  • A new treatment that involves ultrasound-guided delivery of alcohol into the enlarged thyroid gland to kill the tissue has been reported, but requires further refinement to demonstrate safety.
  • Beta-blockers (atenolol) are sometimes used for medical therapy when a cat cannot tolerate Methimazole treatment. This therapy blocks some of the effects of thyroid hormone excess, but it is not an ideal treatment.
  • Dietary therapy with Hill’s Prescription Diet y/d may be an option in some cats. The diet is low in iodine and may improve thyroid health when fed as the sole food. It may be recommended in pets with mild disease or in conjunction with other treatment methods. Hill’s research suggests that y/d may improve thyroid health in 3 weeks.
  • Concurrent congestive heart failure is sometimes found and requires separate appropriate treatment.
  • Follow-up Care for Cats with Hyperthyroidism

    Optimal treatment for your cat requires a combination of home and professional veterinary care. Follow-up can be critical.

    Administer prescribed medication as directed and be certain to alert your veterinarian if you are experiencing problems treating your cat. Optimal follow-up veterinary care for hyperthyroidism often involves the following:

  • Cats receiving Methimazole should have periodic examinations and blood tests to monitor the effects of this drug on the bone marrow and to evaluate for liver toxicity. This is especially important if your cat acts ill, loses his appetite or is lethargic.
  • Hyperthyroidism can mask underling kidney failure. Blood tests post treatment are indicated to look for underlying kidney dysfunction. For more information – go to: Hyperthyroidism and The Kidney.
  • Follow-up is rarely required for cats treated with radioactive iodine.
  • Cats treated by unilateral thyroidectomy need to be re-evaluated because it is common for the gland on the opposite side to become affected at some future time. 
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